1. Field of the Invention
The present invention relates to a NOx-decomposing electrode, i.e., an electrode for decomposing or reducing NOx, especially for decomposing NOx to produce oxygen, and to a NOx concentration-measuring apparatus for measuring NOx contained in the atmospheric air or in the exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
A method has been hitherto known, in which NOx contained in a measurement gas is detected by measuring an electromotive force generated on a NOx-decomposing electrode when an exhaust gas discharged from a vehicle or an automobile is introduced as the measurement gas into a sensor which includes the NOx-decomposing electrode (see Japanese Laid-Open Patent Publication No. 2000-171436). In this technique, the NOx-decomposing electrode is a cermet electrode composed of an alloy of Pt—Rh and a ceramic component. The NOx-decomposing electrode is formed on an oxygen ion-conductive solid electrolyte such as zirconia.
A ratio between Pt and Rh (Pt:Rh) in the NOx-decomposing electrode ranges from 20:80 to 1:99 or from 10:90 to 1:99 by weight ratio. When above ratio is adopted, the oxidation reaction and the reduction reaction of Rh are suppressed on the NOx-decomposing electrode. Further, the adsorption of NOx to the NOx-decomposing electrode, which is used at a low temperature, is suppressed, and the alloy formation between the NOx-decomposing electrode and another metal element (for example, Au) is suppressed. Therefore, a NOx concentration-measuring apparatus, which uses the NOx-decomposing electrode as described above, the stabilization of the impedance of the pumping cell and the stabilization of the sensitivity of measurement of NOx.
It is demanded for such a NOx-decomposing electrode that the ability to decompose NOx is high and the oxidation reaction and the reduction reaction can be suppressed. In such a case, the ability to decompose NOx is improved by increasing a ratio of Rh in the Pt—Rh alloy so that the ratio is larger than a ratio of Pt in the Pt—Rh alloy. On the other hand, the oxidation reaction and the reduction reaction are suppressed by increasing the ratio of Pt in the Pt—Rh alloy so that the ratio of Pt in the Pt—Rh alloy is larger than the ratio of Rh in the Pt—Rh alloy. Therefore, it is impossible to realize the improvement of the ability to decompose NOx and the suppression of the oxidation reaction and the reduction reaction in the NOx-decomposing electrode as described above.
Further, the NOx-decomposing electrode measures NOx at a high temperature of 700° C. to 800° C. Therefore, the NOx-decomposing electrode repeats thermal expansion and thermal contraction every time when NOx is measured. As a result, the NOx-decomposing electrode is exfoliated from the solid electrolyte, and the ability of the NOx concentration-measuring apparatus to measure NOx is lowered.